Disposal of waste cyanide



DISPOSAL Patented Aug. 29, 95o

2,520,703 V or WASTE CYANIDE Robert M. Wagner, Anderson, Ind., assignor to General Motors Corporation, Detroit, Mich, a

corporation of Delaware No Drawing. Application June 14, 1946, Serial No. 676,865

'e'ntrains a considerable amount .of the plating solutionwhich contains sodium and/or potassium cyanide, copper cyanide and sodium hydroxide. In some cases it is necessary to dispose of entire plating solutions containing the above mentioned constituents, which have been evaporated and which because of high carbonate content are not consideredsuitable for: plating-use. Zinc baths and. sludges therefrom contain zinc cyanide, so-

dium cyanide, sodium hydroxide and sodium carbonate. I r

Various methods have been employed or have been proposed heretofore for disposing of waste cyaniderrom-cyanide plating baths. All.of the prior methods of which I am aware are subject to one or more objections.

For example, one method of disposing of waste cyanide is by controlled acid treatment under ventilated stacks thereby dissipating the resulting HCN high in the air. This method does'not take care of metallic cyanides such as copper, zinc, etc., which are precipitated by the acid treatment. It has been proposed, also, to treat the waste with calcium hypochlorite (bleaching powder). However, this method is expensive and requires large quantities of calcium hypochlorite. Another proposed method is treatment with lime sulphur. This method i unsatisfactory because reaction is not complete. It is also quite expensive and impractical to treat large quantities of waste cyanide material in this way. Other proposed methods are to burn the solid material in out-of-the-way places, or to run it into openpits from which it would be absorbed and difiused into the earth. These procedures are objectionable because of the danger of contaminating underground water supplies.

The primary object of this invention is to provide an improved, simple and practical method of disposing of the residue from cyanide plating tanks as well as to dispose of entire plating baths of the cyanide-type which are no longer to be used. Other cyanides such as those used in heat treating operations (barium cyanide, sodium cyanide, potassium cyanide, etc.) sometimes have to be disposed of and it is also an object of the 5 Claims. (Cl. 204-106) 2 present invention to provide an improved,jsimple and practical method of disposing of such waste cyanides. I have found that the waste cyanide material can be treated by electrolyzing a heated aqueous solution of the cyanide-bearing material using insoluble anodes, and continuing the electrolyzing action until the cyanide is reduced to a negligible amount. I

The desired high temperature of the solution can be had by means of suitable heating means in the bath such as, for example, steam coils. A bath temperature of 200 F. has proven quite satisfactory and in practice is obtained with simple heating equipment. Higher temperatures accelerate decomposition. Of course, boiling will cause slopping over of solution. The process proceeds more slowly at temperatures lower than The solution composition is not critical and may range from a very low figure to the limit of solu bility, of the material being destroyed) In dis-'- posing of cyanide residues the process can be carried outin simple manner by. merely dissolving all the material possible in the tank which is used leaving some excess on the bottom. This residue later goes into solution as metal is plated out and cyanide is destroyed.

The current density employed can vary over a wide range as it is not critical. In certain applications a D. C. generator having a voltage of about l3volts has been employed. In these cases the initial cathode current density has been about amperes per square foot. Anode polarization and depletion of free cyanide cause this current density to drop off to about 20 amperes per square foot as decomposition progresses. Improved results can be obtained by using somewhat lower initial current density to plate out all metal possible before the free cyanide is decomposed. However, the procedure first described requires a minimum of attention and control.

Speed of decomposition decreases as the process proceeds, being rapid at first and slow for the last traces of cyanide. Analyses for metal (copper, zinc, etc. as the case may be), free cyanide (titration), and total cyanide (by distillation) are made at frequent intervals, and the solution may be discarded whenever the metal and free cyanide are zero and the "total cyanide reaches such a low figure that its subsequent dilution in the sewer renders it unimportant. This total cyanide is probably in the form of ferri-, or ferro-cyanide and is not generally regarded toxic.

. ment can be .carriedout inside la building. r

Any suitable insoluble anodes are used. In a large installation anodes and cathodes of hea y expanded metal, steel, for example, have been a used.

In one specific example involving a large amount of wast material from copper cyanide plating baths, ;,a large tank was installed in the open to eliminate thezneed for anaexhaust system. A steam coil was installed in the tank and .waste salts and solutions were dissolved in this tank. The solution temperature was maintained at 200 Frand a generator having-a voltage-of about 13 volts connected across the tank with anodesand cathodes made of heavyeexpanded metal.

represents the limit of sensitivit of the distillation method of determination. The timemeauired ranged from several hours to three or four days depending, .of .eourse, --.upon the original concentrationeoi solution. Air agitation .ean the emp'loy'ed'for the purpose of speeding up thetprocess;

By use of a suitable exhaust system the treat By use of the "procedure described herein large amounts of. waste material iromr metal cyanide plating .ibaths. have lheen destroyed. The process destroys both the free cyanide and the -metal exanide of .the plating bath. .Cllhemetal tha'lt'is plated on the rceithodelin the process tof sdestroy- It was found that under the above con- 1 V ditions the cyanide concentration could be re- I duced to less than four parts per mill-ion'which solution of the waste copper cyanide material at a temperature of about 200 F. until the copper and free cyanide are substantially zero and the solution is substantially non-toxic.

3. The process of treating waste cyanide mate'- rial from zinc cyanide plating baths which comprises epassing an ielectric current irom (an insolublefanodezte 'a cathodethrough anaqueous solution of the waste zinc cyanide material at a tem- ;perature of about 200 F. until the zinc and free cyanide are substantially zero and the solution is substantially non-toxic.

4, The processof treating'the residue from the bottoms of metal-cyanide plating baths which ing the was'tecyanide can be recovered anamay t 'he lusedasanodeimatenial inmetal plating baths.

Jiclaim; V. x 1. .The pmcess of treating waste .cyanideematefrom metal cyanide platinebaiths which .complises passinglan electric currentiroman insolulile (anode .t0 ;a voathode through anaqueeus solution of the waste cyanide material at a tempera- .ture of about 200 F. .until the metal and tree am cyanide .arelsuhstantially zero and the solution is sxibs'tantiaLllynon-Ltoxic.

T2. The ,nmcess .01 treating .waste eyanidem'ate: rial Irom copper cyanide platingnbaths which comprises passing an electric current lirom an insoluble .anQde to a cathode through an aqueous "includes dissolving-all of the waste material posat eatemperatureeof .abDutQZQO? IE. iuntil'thetsdlntionissubstantiallynonetoxic. a w

IBQBERT"M.

V REFERENCES'CITED v The followin references are -:-.of record file of this -patent:

UNI-TED STATES PnrrENms Number vName fDate 756,211 Butters Apr. 55,. JHD} 1,5.31;.863 Liebknecht umumwMan. 31,1925

@EBHER FREFEEENGES ChemicalAbstractsfl 1101.27 '1933),, geat1; Products 'Finishingfil eb. 71942,;page48. 

5. THE PROCESS OF TREATING WASTE CYANIDE MATERIAL WHICH INCLUDES PASSING AN ELECTRIC CURRENT FROM AN INSOLUBLE ANODE TO A CATHODE THROUGH AN AQUEOUS SOLUTION OF THE WASTE CYANIDE MATERIAL AT A TEMPERATURE OF ABOUT 200*F. UNTIL THE SOLUTION IS SUBSTANTIALLY NON-TOXIC. 